🧪 [testing improvement] Add error path test for invalid number of target qubits in add_op_to_circuit

Addresses a testing gap in `qsimcirq/qsim_circuit.py` where the `NotImplementedError` for controlled gates with more than 4 target qubits was not explicitly tested.

🎯 **What:** The testing gap addressed
- Added `test_add_op_to_circuit_too_many_targets` to `qsimcirq_tests/qsimcirq_test.py`.

📊 **Coverage:** What scenarios are now tested
- Controlled gates with more than 4 target qubits (specifically 5 targets) are now verified to raise `NotImplementedError` with the correct error message.

✨ **Result:** The improvement in test coverage
- Increased reliability of the `add_op_to_circuit` function by covering a previously untested error path.

Author: mhucka

qsimcirq_tests/qsimcirq_test.py

@@ -879,6 +879,23 @@ def test_control_limits():
         _ = qsimSim.simulate(cirq.Circuit(CHHHHH), qubit_order=qubits)
 
 
+def test_add_op_to_circuit_too_many_targets():
+    # qsim supports up to 4 target qubits for controlled gates.
+    qubits = cirq.LineQubit.range(6)
+    # A gate with 5 targets and 1 control.
+    big_gate = (
+        cirq.MatrixGate(np.eye(32)).on(*qubits[1:]).controlled_by(qubits[0])
+    )
+
+    qsim_circuit = qsimcirq.qsim.Circuit()
+    qubit_to_index = {q: i for i, q in enumerate(qubits)}
+
+    with pytest.raises(
+        NotImplementedError, match="Received control gate on 5 target qubits"
+    ):
+        qsimcirq.add_op_to_circuit(big_gate, 0, qubit_to_index, qsim_circuit)
+
+
 def test_decomposable_gate():
     qubits = cirq.LineQubit.range(4)